I think ideally, you want something which is non-conductive, incompressible, and solid but soft. A solid material will not flow and potentially leak through the

I think ideally, you want something which is non-conductive, incompressible, and solid but soft. A solid material will not flow and potentially leak through the seal. A soft material will not fracture, and transmit stress to the components of the watch. In a hard material, fractures might also appear when thermally shocked (such as getting in and out of the water). I'd suggest a good candidate would be a silicone gel. It can be applied to the watch in liquid form, allowing all air to vacate, then cure into a soft solid once sealed in the watch. It could also be removed relatively easily, if you decided you don't want it in there any more.